Re: [Tarantool-patches] [PATCH luajit 1/2] Add support for full-range 64 bit lightuserdata.

[Sergey Kaplun via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Igor,

On 01.08.21, Igor Munkin wrote:
> Sergey,
> 
> Thanks for the fixes and comments! LGTM then, except the three typos
> I've missed in the previous review.
> 
> On 28.07.21, Sergey Kaplun wrote:
> > Igor,
> > Thanks for the review!
> > 
> 
> <snipped>
> 
> > 
> > The new commit message is the following:
> > 
> > ===================================================================
> > Add support for full-range 64 bit lightuserdata.
> > 
> > (cherry picked from commit e9af1abec542e6f9851ff2368e7f196b6382a44c)
> > 
> > LuaJIT uses special NaN-tagging technique to store internal type on
> > the Lua stack. In case LJ_GC64 first 13 bits are set in special NaN
> > type (0xfff8...). FPU generates the only one type. The next 4 bits are
> > used for an internal LuaJIT type of object on stack. The next 47 bits
> > are used for storing this object's content. For userdata, it is its
> > address. In case arm64 the pointer can have more than 47 significant
> > bits [1]. In this case the error BADLU error is raised.
> > 
> > For the support of full 64-bit range lightuserdata pointers two new
> > fields in GCState are added:
> > 
> > `lightudseg` - vector of segments of lightuserdata. Each element keeps
> > 32-bit value. 25 MSB equal to MSB of lightuserdata address, the rest are
> > filled with zeros. The lentgh of the vector is power of 2.
> 
> Typo: s/lentgh/length/.

Fixed.

> 
> > 
> > `lightudnum` - the length - 1 of aforementioned vector (up to 255).
> > 
> > When lightuserdata is pushed on the stack, if its segment is not stored
> > in vector new value is appended on top of this vector. The maximum
> > amount of segments is 256. BADLU error is raised in case when user tried
> 
> Typo: s/tried/tries/.

Fixed.

> 
> > to add userdata with the new 257-th segment, so the whole VA-space
> > isn't covered by this patch.
> > 
> > Also, in this patch all internal usage of lightuserdata (for hooks,
> > profilers, built-in package, IR and so on) is changed to special values
> > on Lua Stack.
> > 
> > Also, conversion of TValue to FFI C type with store is no longer
> > compiled for lightuserdata.
> > 
> > [1]: https://www.kernel.org/doc/html/latest/arm64/memory.html
> > 
> > Sergey Kaplun:
> > * added the description and the test for the problem
> > 
> > Resolves tarantool/tarantool#2712
> > Needed for tarantool/tarantool#6154
> 
> Minor: Why #5629 is not mentioned?

Added.

> 
> > ===================================================================
> > 
> 
> <snipped>
> 
> > 
> > See the iterative patch below.
> > 
> > ===================================================================
> 
> <snipped>
> 
> > diff --git a/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c b/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c
> > index 801c7fe1..c958841f 100644
> > --- a/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c
> > +++ b/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c
> 
> <snipped>
> 
> > @@ -26,8 +34,8 @@ static int longptr(lua_State *L)
> >  	 * equals -1.
> 
> Typo: s/hint equals -1/hint that equals to -1/.

Fixed:
===================================================================
diff --git a/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c b/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c
index c958841f..1b909fc6 100644
--- a/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c
+++ b/test/tarantool-tests/lj-49-bad-lightuserdata/testlightuserdata.c
@@ -30,8 +30,8 @@ static int mmaped_ptr(lua_State *L)
         /*
          * If start mapping address is not NULL, then the kernel
          * takes it as a hint about where to place the mapping, so
-         * we try to get the highest memory address by hint
-         * equals -1.
+         * we try to get the highest memory address by hint, that
+         * equals to -1.
          */
         const size_t pagesize = getpagesize();
         void *mmaped = mmap(START, pagesize, PROT_NONE, MAP_PRIVATE | MAP_ANON,
===================================================================

> 
> >  	 */
> >  	const size_t pagesize = getpagesize();
> > -	void *mmaped = mmap(START, pagesize, PROT_NONE,
> > -				  MAP_PRIVATE | MAP_ANON, -1, 0);
> > +	void *mmaped = mmap(START, pagesize, PROT_NONE, MAP_PRIVATE | MAP_ANON,
> > +			    -1, 0);
> >  	if (mmaped != MAP_FAILED) {
> >  		lua_pushlightuserdata(L, mmaped);
> >  		assert(mmaped == lua_topointer(L, -1));
> 
> <snipped>
> 
> > ===================================================================
> > 
> 
> <snipped>
> 
> > 
> > -- 
> > Best regards,
> > Sergey Kaplun
> 
> -- 
> Best regards,
> IM

-- 
Best regards,
Sergey Kaplun